.Display top of file and end of file at same time.
      PF2 enable display both on each split screen.
      (one of those should be browse mode).
      X line command also enable that excluding intermediate range.
      Use "Y" line cmd to re-display partially.
      Use "RESet X" cmd to reset all X line.
      use FLIp(or alias XX) cmd to exchange displayed and excluded lines.